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How do fibres improve the fracture toughness of ceramic-matrix composites?
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advanced structural ceramics
Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.
In ceramic-matrix composites, fibres improve fracture toughness primarily by engineering the microstructure to resist crack propagation and absorb energy. This process occurs through two main mechanisms:1
- Crack Deflection: Fibres force advancing cracks to deviate from a straight path. This deflection significantly increases the total length of the crack, which requires more energy to propagate.1
- Crack Bridging: As a crack moves through the material, fibres located behind the crack tip span the opening, effectively "bridging" the gap. This exerts a closing force on the crack, which helps to hold it together and hinders further growth.1
These mechanisms result in a greater fracture surface area and increased energy absorption, allowing fibre-reinforced composites to achieve significantly higher fracture toughness values compared to conventional, brittle ceramics.1